"The same cause explains why the land is not visible from the deck of a ship when in sight from the mast-head; and why as a vessel passes far into the distance, if some shining object is tied to the top of the mast it appears slowly to sink and finally it is hidden from sight."

Content from external source

People who believe the Earth is flat (or sometimes just that local bodies of water are flat) are big fans of method B (distant light). Furthermore, they prefer to use a laser rather than a flashlight. They also like to do the test at night, rather than in the day. While there are some slight potential advantages to this method, I think their focus on the laser method is fundamentally flawed and results in a lot of wasted effort and unfounded conclusions.

Let's divide the methods into three and look at the pros and cons of each one. The three methods are:

Daylight observations of a distant shore

Night observations of a powerful light

Night observations of a laser

1. Daylight observations of a distant shore

Pros: You can clearly see what is going on. Don't need anyone on the other side. Compelling photos and videos.Cons: Does not work at night.

The above image shows the Malibu Beach Inn viewed from Santa Monica. The two images are taken from different heights above sea level, but in both images, the beach and a lower part of the hotel are obscured by the horizon. You can quite easily see here that if someone had a powerful light and was standing on the beach, then that light would also be hidden by the horizon.

The further you go, the more apparent this is. From the same location, I took a photo of Point Dume (a headland to the northwest of Malibu). I overlaid the photo with what Point Dume actually looks like from closer up, to show how much is hidden with the camera about two feet above the water:
Point Dume is about 200 feet above sea level, 15 miles from Santa Monica, and more than half of it is hidden behind the curve. So again if there was someone standing on the beach waving a flashlight or a laser, then they would be about 100 feet below the horizon and totally invisible.

So why, given these inarguable demonstrations in daylight, why would you want to use a flashlight or a laser? There are actually some benefits to using a light, but there's also a big negative - sometimes refraction will make the light visible. But let's look at the two other methods:

2. Night observations of a powerful light

Pros: Works at night (and sometimes during daytime). Tells you if there's a line of sight from the camera to the light. Does not need magnification. Easy to aim. Relatively cheap.Cons: Night observations don't tell you if the line of sight is straight. Can't really tell what is going on if you can't find the light. Can't verify where the light actually is.

In this experiment, you stand on one shore with a camera while a friend stands on the other with a powerful flashlight pointed in your general direction. You'd generally communicate by phone and they would flash the light on and off so you can distinguish it from other lights.

If you can see the light then that means there's a line of sight from the light to the camera. This is good in that it allows you to identify the visibility of one particular spot, but it's terrible in that you've got no idea why that particular spot is visible.

The problem, of course, is refraction. When looking at the horizon from a low elevation (a few feet above the water) the effects of the cool air directly over the cold water can often bend light downwards. This has the effect of visually flattening out the curve of the earth slightly. But quite often there a narrow band just grazing the water where a lot of the distant scene is compressed together.

The beauty of doing a daylight observation is that you can see when this is happening. When there is significant additional refraction the view is distorted and highly compressed near the horizon. A good example of this is the view of Toronto from Fort Niagra by @jenna1789

Here we can see that the white arched building has been lifted up over the horizon, but has been terribly distorted and compressed (as have the other buildings, looking rather short and stubby). And really you can't see the base of it.

However, we can see in this extreme example that may 100 feet is "visible" when it should not be. So it's quite possible that a light or a laser could also be made visible.

But, and here's the key problem, you can't see if it's due to refraction, because it's at night!

Think of it another way. With a flashlight, you are just getting data about one particular spot. In daylight, you are getting data about all the spots. You actually see what is going on around any particular point and if it has been lofted by refraction. With a flashlight at night, you can't tell.

3. Night observations of a laser

Pros: Works at night and during dimmer parts of the day. Laser is more visible if aimed at the camera, can see where it hits if the beam does not spread out too much, can take measurements of beam height along the path, you can see a portion of the beam. Can "level" the beam.Cons: Powerful narrow beam lasers are expensive and fiddly. Risk of blindness. Difficult to aim. Night observations don't tell you if the line of sight is straight. Can't really tell what is going on if you can't find the light. Can't verify where the light actually is.

A laser suffers from all the same problems as a bright flashlight, and if you are simply wanting to test if you can see something from one position it really not an improvement. It does have a theoretical advantage in that when it is aimed directly at the camera it is significantly brighter than a comparably powered flashlight. However power is not a huge problem - you can quite easily get a 3500 lumen flashlight that's visible from 10-20 miles away (at night).

The only real advantage of a laser is that while the beam is still narrow, you can tell where the center is. You can then use this to take measurements along the route, and hence figure out the shape of the path of the laser relative to the water surface.

In the above example, they are marking the position of the laser near the shore, and then a few miles out. By that point, the laser beam has spread out, but you can still see the approximate shape. This does not work beyond a few miles though, as it's impossible to keep a tight beam over long distances. It's also problematic if you only take two readings, as the beam might be tilted upwards very slightly. Ideally, you would take several readings and plot them on a graph to show the curve.

So if all you are doing is trying to see if there's a line of sight at night, then use a powerful flashlight. It's a lot simpler.

However you've really got to ask yourself if there's a benefit to performing the experiment in the dark. Why not use daylight observations, which gives you vastly more data to work with?

One reason to prefer night observation is that it's cooler, so the air temperature is closer to the water temperature. However, if that is the reason, the simpler approach would be to start the observations at dawn, when the air was coolest, and continue to record visible observations of the distant shore as the day warmed up. Ideally, this would be a timelapse close-up video of the distant shore.

A fourth/fifth method is daytime observation of a laser or powerful light, as planned for the 2019 pier2pier observation. A sixth method is observing the light produced by reflecting the sun with a mirror.

There is regularly a big difference between refraction during the day (low/negative k), and during evening/night/early morning (rising/big k). The placement of the light and the camera is also very important: placing them close to the surface can incur some very strong refraction. Observing across water also usually amplifies refraction.

Note that choosing to observe in the evening or at night and low over water is likely to lead to a great amount of refraction, making the Earth appear much flatter than it is. Absence of wind (calm) also leads to more stable inversion layers with higher refraction; wind mixes up the atmosphere and reduces thermal gradients. The best opportunity for unrefracted observations is probably 9:00 to 15:00 on a cloudy windy day with sight lines high over land.

Are all sources of light subject to refraction, eg including laser pointers etc.

Also how would a laser pointer behave when set up with a spirit level, close to the water. Would it not actually 'point upwards' when viewed from a short distance away, in example the upcoming pier to pier observation by Dr John D. In other words what kind of results could be expected from such an observation.

Also how would a laser pointer behave when set up with a spirit level, close to the water. Would it not actually 'point upwards' when viewed from a short distance away, in example the upcoming pier to pier observation by Dr John D. In other words what kind of results could be expected from such an observation.

Click to expand...

If a laser was leveled at its source, then it will seem to "rise" up away from the curve of the earth. However it's hard to see this, as the laser beam spreads out so as to be invisible after a relatively short distance. In the Brighton "pier-to-pier" experiment the lasers being used appear to be $10 generic lasers with no special optics, so the beam will quickly diverge.

What will likely happen is that they will see the light from the laser at some distance (when looking back at the laser), due to refraction effects. There's a number of variables.

Are all sources of light subject to refraction, eg including laser pointers etc.

Also how would a laser pointer behave when set up with a spirit level, close to the water. Would it not actually 'point upwards' when viewed from a short distance away, in example the upcoming pier to pier observation by Dr John D. In other words what kind of results could be expected from such an observation.

Click to expand...

The observation takes place (starts) an hour before sunset with the sun low in the sky, and the air starting to cool. If the weather was overcast and windy, I would expect the refraction to be slight, and the lights only to be visible from a height; if the day was sunny and the evening is calm, I would expect strong refraction with better visibility between the observation points. The bending of the light makes the other side "rise" visually, so the laser would not necessarily look bent.
In any case, refraction should strengthen as night falls.

Here is video of refraction making an obstructed light visible and enlarging it considerably, with a "garden shed" setup using butane gas to manipulate the refractivity of the air. The refraction sequence starts at 0:35.

Normal refraction (at least over land) will normally increase at night, for a few reasons.
This paper "Estimation of temporal variations in path-averaged atmospheric refractive index gradient from time-lapse imagery" https://doi.org/10.1117/1.OE.55.9.090503 showed that refraction increased at night, causing a distant building to appear higher at night.
They graphed the apparent vertical displacement, and the peaks were at night, with minimums in the afternoon.
(Thanks to @Mendel for pointing out this paper).

I've replicated this experiment myself over one day. The house here is about 4 miles away, and 100 feet higher than me, but the effect is still visible.

However, that's over land. It's not clear what happens over water. Water holds its heat longer than ground and vegetation does. Over land, the land tends to cool faster than the air. But over water, the water may well end up warmer than the air.

However, that's over land. It's not clear what happens over water. Water holds its heat longer than ground and vegetation does. Over land, the land tends to cool faster than the air. But over water, the water may well end up warmer than the air.

Click to expand...

sorry, i thought he was speaking specifically about Brighton next week at 3pm. Although my first source was saying the sea is 2 dgrees C colder than the air. And i didn't double check because intuitively it sounded right. (i dont live by the ocean).

Actual sea surface water temperatures close to shore at Brighton can vary by several degrees compared with these open water averages. This is especially true after heavy rain, close to river mouths or after long periods of strong offshore winds. Offshore winds cause colder deep water to replace surface water that has been warmed by the sun

However, that's over land. It's not clear what happens over water. Water holds its heat longer than ground and vegetation does. Over land, the land tends to cool faster than the air. But over water, the water may well end up warmer than the air.

Click to expand...

sometime in the afternoon, the sun stops heating the atmosphere, and it starts to cool, get denser, and compress downward. That increases the pressure and density gradients, and causes evening dew.
Over land, the surface cools as well, but water cools a lot less, resulting in a steeper temperature gradient, too. (...)

The effects of refraction vary more and more the closer you are to the ground, which means experiments that are close to the ground have variable results. This all should be fairly obvious for observation and has been studied in some depth.

For the higher atmosphere, some 100 m above the ground and higher, the vertical temperature gradient is fairly independent of the temperature of Earth’s surface. . The vertical temperature gradient ∂T/∂z is about −0.006 K/m, i.e., a temperature decrease of 6 K per km height
...The intermediate atmosphere, about 20–30 m to some 100 m [cf. Webb, 1984; Wunderlich, 1985], is weakly influenced by the temperature of the surface and characterized by temperature gradients frequently of about −0.01 K/m,
...
In contrast to the higher and intermediate atmosphere, the thermal characteristics of the air strata of the lower atmosphere (lowest 20–30 m) are strongly subjected to the varying thermal properties of the surface [e.g., Angus‐ Leppan, 1969]. In essence, two processes of heat transfer govern the temperature gradients occurring in this region ...
1. Over the day, Sun radiation is being absorbed by Earth’s surface. The warm terrain, in turn, heats up the lowest
atmospheric layers, resulting in negative vertical temperature gradients, and turbulent motions of air ...
2. In the evening, the Earth’s surface normally cools off faster than the overlaying air strata. Usually, this results in strong positive gradients

Content from external source

Note all these cases are more than 20m (65 feet) above the ground, and there's still considerable variation. My observations above were from ridge to ridge with 200 feet of valley below, and another ridge in the middle. But since it's all relatively high, it's fairly simple. Closer to the ground is more complex.

For ground clearances of 1–3 m, [Brocks] showed that the refraction coefficients may exhibit extreme values between −3.5 and +3.5.
...
Brocks [1950b] demonstrated that refraction effects in the lower atmosphere generally multiply with decreasing height. This is because heat transition from the Earth’s surface is the stronger, the less distant the atmospheric layers are. Therefore, the largest absolute values of temperature gradients are to be expected immediately above the ground.

Content from external source

There's little discussion of over-water observations, and none at night at very low altitudes

[ice and water] significantly differ from vegetated ground in terms of their thermal storage properties, usually resulting in an amplification of refraction effects.

Content from external source

What we are primarily interested in here is the difference between night and day refraction over water at very low altitudes - i.e. the altitudes of flashlight and laser tests.

While Hirt says "an amplification of refraction effects" this clearly cannot mean "like over ground, but more so." He says "the Earth’s surface normally cools off faster than the overlaying air strata", but water maintains its temperature, cooling slower than the overlaying air.

Boiling this down, the question being asked is often "can refraction account for these observations?" Typically this question is accompanies with some spot measurements of air and water temperatures. But these measurements tell us very little about the temperature profile over the bulk of the light path. As Professor Andy Young noted elsewhere, the observations of refraction often tell you more about the air mass than measurements of the air mass will predict the refraction.

Yes, although the amount of refraction varies slightly by wavelength (color). Lasers are still visible light, and will be refracted basically exactly the same as daylight.

Click to expand...

I dare say this is also true for infra red, there is a new trend of using infra red cameras that flat earth proponents are using as evidence of a flat earth.

As you mention that the laser used will be of cheap generic variety, which will diverge as the light travels, it seems that the lasers will be mounted at the back in a 1 meter metal tube, there then being a rubber cap with a hole in it for the light to exit, it is possible they are doing that to minimise divergence. As an astronomer I see a potential flaw in that as the metal would be colder than the internal air, I use a 'dew shield' on the front of my telescope to combat that along with leaving out a few hours before viewing.

I dare say this is also true for infra red, there is a new trend of using infra red cameras that flat earth proponents are using as evidence of a flat earth.

Click to expand...

Yes, there's very little difference between visible light (average 550nm) and infrared light (>750 nm). Even going into LWIR at 15,000 nm you get the same basic refraction effects.
I'll add a wavelength input to the simulator